Fix crash when no post processor is defined

Exception:

> ContextErrorException: Notice: Undefined index: post_processors in /var/www/foo/vendor/liip/imagine-bundle/Liip/ImagineBundle/Imagine/Filter/FilterManager.php line 144

This happens in eZ Publish, where post processors are not (yet) supported.

Ping @crevillo
I meant to fix it in LIipImagineBundle, not your code. The configuration or di extnesion of this bundle. The place where filter manager service is configured.